1. Field of the Invention
The present invention relates to an image processing technique for effectively suppressing color banding.
2. Description of the Related Art
There have been proposed home entertainment systems capable of not only executing game programs but also playing moving images. In such home entertainment systems, GPUs generate three-dimensional images using polygons (see Patent document 1, for example).
When smooth gradation with little noise is displayed on a display using gradation expression of an image format as typified by JPEG, there occurs “color banding,” by which stair-like boundaries emphatically appear because of visual characteristics. Since general image formats, such as JPEG, MPEG, and BD formats, only support low gradation of 8-bit gradation, displaying a gradation image with low noise will cause color banding in principle. In order to prevent such color banding, there have been conventionally proposed methods such as dithering, in which noise is intentionally added to image data, and an error diffusion method, in which granular noise is added.
[Patent Document 1] U.S. Pat. No. 6,563,999
However, such noise as added to reduce color banding also causes reduction of image compression ratio. Also, even though noise is added to image data, color banding may often occur as a result because the image processing performed thereafter alters or transforms the added noise, thereby preventing the noise from performing the intended function.
Meanwhile, there have been developed in recent years display interfaces as typified by the HDMI (High-Definition Multimedia Interface) standard, which tend to support a larger number of bits. For example, with version 1.3, the HDMI standard defines signal transmission for 12 bits×3 colors, so that HDTV images with high gradation have become familiar. However, since images are quantized to 8 bits in an image format such as JPEG or MPEG, there are situations in which high-definition display interfaces are not fully utilized.